The need for all plastic containers and cans has been recognized more and more as environmental concerns grow and new technologies are finding acceptance in the marketplace. For instance, recycling of packaging materials has imposed restrictions on those packages consisting of more than a single material wherein the various components cannot be easily separated in the recycling process. As a result, cans of aluminum are considered satisfactory but cans of plastic which are closed by metal ends or reinforcing members are not.
Also the advent of oven heating employing microwave energy introduces another problem to all-metal cans or to plastic cans with metal ends in that the metal becomes very hot and may build up a high electrical potential which can discharge or spark to other metallic surfaces including the oven walls.
Additional problems exist with food products which are heated in plastic packages during autoclaving related to the distortion of the walls or ends resulting from the higher temperatures and internal pressure buildup Since the package typically cannot be vented, care must be taken to provide and adequately control the external pressure to balance the internal pressure buildup and when concave can ends are bowed outwardly, another operation of bowing them back to their original shape may be required.
Prior to the present invention, moreover, techniques used to join metal can ends to can bodies of metal generally have been directed to rolling their free ends together sequentially around their perimeter. In one method the rolling tool first sequentially and circumferentially curls the seamed free ends together, followed by a flattening or squeezing operation to bring the curled surfaces into a more intimate engagement suitable for sealing. Without the latter operation the curls produced are not tight enough to make a good seal. In another method rolling tools directly but still sequentially and circumferentially fold the free ends in separate operations into a tight flattened shape suitable for sealing without a prior curling operation.
In one effort to form plastic cans Heider et al (U.S. Pat. No. 3,543,963) discloses a seamed plastic can end and plastic can body sequentially and circumferentially folded, bent and reshaped by a sequential rolling and folding process. This is relatively complicated and cumbersome and can be time consuming. Further it appears the technique proposed by the Heider et al in 1970 has not met with commercial acceptance.
In another effort, at about the same time, Shelby et al. (U.S. Pat. No. 3,428,238) disclosed a seamed plastic can end and plastic can body. In the Shelby et al. technique the curled seam produced between the plastic can body and plastic end portion, among other things, does not possess reliability and long term integrity, nor can they be produced economically in high speed seaming operations. In Shelby et al. the ends forming the seam are preheated to temperatures above the softening temperature of the materials employed which produce relatively limp plastic ends substantially devoid of elastic memory that are then reshaped. By this method, the curled ends of the Shelby et al. seam do not have the dynamic properties necessary for producing plastic seams of integrity and reliability especially for use with products under pressure or vacuum. Like Heider et al., the Shelby et al. technique proposed in 1969 has not met with commercial acceptance.
Rather, those in the art have had to resort to using metal can ends seamed to the free ends of plastic container bodies by folding, bending and reshaping the ends with sequential and circumferential rolling techniques.
Thus, there is a persisting need to provide satisfactory containers and cans comprising plastics, particularly at the seams formed by an end and adjacent body portion of the container which can withstand rigorous commercial application.